1. Field of the Invention
The present invention relates to a method of depositing layers of semi-insulating gallium arsenide and more particularly to a metal-organic vapor phase epitaxy process for depositing oxygen doped gallium arsenide on a substrate.
2. Description of the Prior Art
The formation of epitaxial layers of semi-insulating gallium arsenide is highly desirable for many device applications, such as MESFETS. Semi-insulating behavior in bulk gallium arsenide is obtained by the introduction of either a native defect, EL2, or a deep level impurity such as chromium. However, high quality semi-insulating gallium arsenide bulk substrates are expensive. In addition, it is desirable to form the semi-insulating layer by using the same basic thin film or epitaxial techniques as are used to form the semiconductor device material.
In epitaxial films, semi-insulating behavior has been obtained in gallium arsenide only when a transition metal has been introduced into the growing layer. Akiyama et al. in J. Crystal Growth 68(1984) disclose incorporation of chromium and vanadium in a metal-organic vapor phase epitaxy system to create semi-insulating gallium arsenide. However, transition metals tend to diffuse quickly through the gallium arsenide resulting in undesired impurity redistribution. The vanadium source used by Akiyama et al. was triethoxyvanadyl (TEV): VO.sub.2 (OC.sub.2 H.sub.5).sub.3 and the authors surmised that the oxygen in the compound may also have contributed to the formation of the deep levels.
It is known in the art that oxygen can create a deep level in gallium arsenide. Lagowski et al. in Appl. Phys. Lett. 44(3) 1984, disclose the formation of an oxygen deep level close to the EL2 level in energy by introducing Ga.sub.2 O.sub.3 into a Bridgeman-type apparatus to form oxygen doped gallium arsenide. However, attempts to form oxygen deep levels in epitaxial gallium arsenide have been unsuccessful, as shown by Ruby et al., J. Appl. Phys. 58(2) 1985, in which the introduction of pure oxygen in a AsCl.sub.3 vapor phase epitaxy system did not result in an oxygen deep level, and by Wallis, Inst. Phys. Conf. Ser. No. 56: Chapt. 1 (1981) in which the introduction of oxygen from water vapor into a metal-organic vapor phase epitaxy system resulted in no oxygen being incorporated during the growth of gallium arsenide. These attempts have not led to oxygen incorporation since the formation of either gallium or arsenic oxides is thermodynamically unfavorable under the typical metal-organic vapor phase epitaxy conditions of high temperatures and in a hydrogen carrier gas. See, Kuech et al., J. Appl. Phys. 62 (1987). However, U.S. Pat. No. 4,253,887 to Jolly, discloses the formation of oxygen doped gallium arsenide in an AsH.sub.3 /GaCl.sub.3 vapor phase epitaxy system by the introduction of water vapor, the results of which are apparently limited to the AsH.sub.3 /GaCl.sub.3 system in view of the teachings of Ruby et al. noted above.
There have been successful attempts of forming oxygen doped semi-insulating aluminum gallium arsenide. Terao et al., J. Crystal Growth 68 (1984) disclose that the introduction of H.sub.2 O or O.sub.2 into an metal-organic vapor phase epitaxy reactor in which gallium arsenide is being grown by trimethyl gallium, trimethyl aluminum and arsine will result in oxygen being incorporated into the growing gallium arsenide. However, there remains a need for a method to form oxygen doped gallium arsenide in a metal-organic vapor phase epitaxy system.